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Abstract

The photon-driven substrate-mediated dissociation of N2O on thin MgO films grown on an Ag(100) crystal has been investigated with postirradiation thermal desorption spectroscopy (TDS). After excitation with 248 nm photons, we observe the simultaneous formation of N2 and a high-temperature oxygen species, accompanied by a decrease in the parent N2O signal. On the basis of the generation and depletion of N2 and N2O as a function of the photon dose, we determine cross sections of about 10–18 and 10–19 cm2, respectively, whereas for the concurrent desorption of the N2 photoproduct a cross section of 10–20 cm2 is found. If the desorption of molecular oxygen is completed at 650 K, then the MgO film is virtually restored to its initial reactivity condition. However, only partial removal of the high-temperature oxygen results in the diminished formation of N2 in subsequent reduction cycles, which we explain by blocking the reactive sites through oxygen at annealing temperatures that are not high enough. Our findings can be rationalized in terms of the initial photogeneration of electron–hole pairs that then upon electron trapping lead to reactive sites that cause the dissociation of the parent N2O molecules.